1. Field of the Invention
The present invention relates to a suspension of a vehicle, such as an automobile, and more particularly, to a hydraulic active suspension adapted to operate with a compensation for side forces due to braking, acceleration and/or turning of the vehicle according to a separate term of calculation.
2. Description of the Prior Art
An example of a known hydraulic active suspension of a vehicle, such as an automobile, is described in, for example, Japanese Patent Laid-open Publication 2-175405. The hydraulic active suspension includes actuators each having a working fluid chamber and provided for a corresponding vehicle wheel to increase or decrease a suspension force of the corresponding vehicle wheel according to an increase or a decrease of the pressure in the working fluid chamber. The suspension system also includes a pressure adjusting means for adjusting the pressure in each of said working fluid chambers, vehicle height detection means for detecting a height of a portion of a vehicle body corresponding to each of the vehicle wheels, an acceleration detection means for detecting the acceleration of the vehicle body, feedback control amount calculation means for calculating feedback control amounts based upon the vehicle heights, feed forward control amount calculation means for calculating feed forward control amounts based upon the acceleration of the vehicle body so as to suppress changes of the posture of the vehicle body due to inertial disturbance to zero, and control means for controlling the pressure adjusting means based upon the feedback control amounts and the feed forward control amounts.
In such an active suspension, the pressure adjusting means, and therefore the pressure in the working fluid chamber of each of the actuators, is controlled according to both of the feedback control amount based upon the vehicle height of the corresponding portion of the vehicle body and the feed forward control amount based upon the acceleration of the vehicle body, whereby a good riding comfortability of the vehicle can be ensured together with an effective prevention of changes of the posture of the vehicle body due to inertial disturbances applied thereto during acceleration, deceleration or turning of the vehicle.
However, in the above-mentioned prior art active suspension, a vertical link reaction force of the suspension due to a longitudinal force acting to the vehicle wheel during braking or acceleration of the vehicle or a lateral force acting to the vehicle wheel during a turning of the vehicle is not taken into consideration, and therefore changes of the posture of the vehicle body, such as a pitching during braking or acceleration or a rolling during a turning, are not always controlled satisfactorily, so that there sometimes occurs a substantial diving or squatting or a lifting up of a side of the vehicle body called a jack-up phenomenon.
Considering, for example, a situation wherein a vehicle is being braked. If the distribution ratio of the braking force between the front vehicle wheels and the rear vehicle wheels is constant, the pitching of the vehicle body will be appropriately controlled by an appropriate adjustment of the gain for determining the rate of increase or decrease of the pressures in the working fluid chambers of the actuators of the front and rear vehicle wheels. However, when the brake system is equipped with a P valve for non lineally changing the ratio of the brake fluid pressure between the front and rear vehicle wheels according to the magnitudes thereof or when the brake system is of an electronic control type which actively controls the distribution ratio of the braking force between the front and rear vehicle wheels, the distribution ratio of the braking force between the front and rear vehicle wheels changes according to the braking conditions.
When, for example, the distribution ratio of the braking force between the front and rear vehicle wheels is adjusted to put an emphasis on the front vehicle wheels during a quick braking in a vehicle equipped with a brake system including a P valve, while the active suspension of the vehicle is of a conventional type in which the pressures of the working fluid chambers of the actuators are controlled in a manner of linear feed forward control according to the longitudinal acceleration of the vehicle, the anti-pitching force for the front vehicle wheels during the quick braking becomes excessive, thereby increasing the vehicle height at the front of the vehicle, while the anti-pitching force for the rear vehicle wheels becomes insufficient, thereby also increasing the vehicle height at the rear of the vehicle, resulting in an increase of the vehicle height at all the four vehicle wheels, which is undesirable not only from the view point of riding comfortableness but also from the view point of dynamics of the vehicle. If the gain for generating the anti-pitching forces is decreased in order to avoid the above-mentioned undesirable phenomenon, there would occur an undesirable phenomenon that the vehicle height at the front vehicle wheels once decreases and then increases, giving the driver an illusion that the control has failed.
On the other hand, when, for example, a vehicle equipped with a drive force distribution control system is accelerated, a phenomenon similar to that described above with respect to the braking occurs. Further, in a vehicle in which only front or rear vehicle wheels are driven, therefore having a hundred percent distribution ratio of the drive force between the front and rear or the rear and front vehicle wheels, even when the gain of the anti-pitching force of the conventional active suspension is adjusted to be optimum for braking, the gain setting will not be appropriate for acceleration, or conversely, if the gain is set to be optimum for acceleration, the gain setting will not be appropriate for braking.
In Japanese Patent Laid-open Publication 4-331616, which is based upon an application, filed by the same applicant as the present application there is described an active suspension in which a jack-up moment is added to the feed forward control amount as a heave control amount to balance a lateral acceleration due to a turning of the vehicle during a constant rate turning, so that a jack-up phenomenon during a turning of the vehicle is thereby suppressed. However, in this prior art active suspension there still are the following two problems.
First, the control amount with regard to the rolling includes therein a role moment sharing component and a link reaction force component, i.e. an anti-roll moment component, mixed with one another. Generally in the vehicle suspensions of passenger automobiles, the anti-roll rate determined by the geometry of the suspension is different with respect to the front vehicle wheels and the rear vehicle wheels, and therefore, when the roll control amount includes a roll moment sharing component and a link reaction force component mixed with one another, if, for example, it is intended to control the ground contact forces of the vehicle wheels by changing the distribution ratio of the roll moment between the front and rear vehicle wheels, it is difficult to precisely control the distribution ratio without changing the total roll moment, which is the sum of the roll moments of the front and rear vehicle wheels. Therefore, there can occur an interference between the control of rolling posture control performance and the control of ground contact force control performance such that when the ground contact force distribution ratio is changed, the rolling of the vehicle body increases.
Secondly, although the afore-mentioned prior art proposes to take into consideration a jack-up suppression control amount according to the lateral acceleration due to a turning of the vehicle, this control amount corresponds to the actual jack-up moment component only during a constant rate turning of the vehicle, but does not correspond to the actual jack-up moment component during a transition into and out of a turning. During a constant rate turning, the sum of the lateral forces applied to the vehicle wheels at the inside and the outside of the turning are substantially proportional to the lateral acceleration applied to the vehicle with respect to the front vehicle wheels as well as the rear vehicle wheels. However, during a transition into and out of a turning of the vehicle, due to the generation of a yaw acceleration, the sum of the lateral forces applied to the vehicle wheels at the inside and the outside of the turning is not proportional to the lateral force with respect to the front vehicle wheels as well as the rear vehicle wheels. Therefore, according to the above-mentioned prior art, the link reaction force components due to the lateral force at each vehicle wheel during a transition into and out of a turning can not be precisely estimated, and therefore changes of the posture of the vehicle wheel during a transition into and out of a turning can not be appropriately suppressed.